Overriding Methods onStartTrackingTouch and onStopTrackingTouch of Interface OnSeekBarChangeListener

Java requires that you override every method in an interface that you implement. This app does not need to know when the user starts moving the slider’s thumb (onStartTrackingTouch) or stops moving it (onStopTrackingTouch), so we simply provide an empty body for each (lines 111–119) to fulfill the interface contract.